Overview.
Basilar invagination is a condition in which the base of the skull around the foramen magnum plunges upward into the cranial cavity, forcing the atlantoaxial vertebrae (odontoid process) below it to elevate into the base of the skull, which can be combined with other bony developmental anomalies (e.g., disorders of vertebral segmentation, disorders of atlantoaxial spine fusion) and neurologic deformities (e.g., Chiari malformation, subcerebellar herniation of the cerebellar tonsils, and spinal cord hydrocephalus), which are seen in 30% of patients. These malformations are seen in 30% of patients with depression of the skull base.
Etiology
The most common cause is a congenital malformation, also known as a flattened skull base. Rarely, it is due to other causes such as bone disease. This is not uncommon in clinical practice. Symptoms can be caused by upward inversion of the bony margins of the skull base in the region of the greater occipital foramen, so that the cricoid vertebrae are plunged upward, and the dentate process of the cardinal vertebrae enters the greater occipital foramen in an upward and forward direction, which compresses the medulla oblongata and the cervical nerve roots.
Types
Cranial base depression often has abnormalities at the base of the cranial base, the squamous part of the occipital bone, and outside the occipital bone, and can be categorized into two types:
1. Anterior change type
The base of the occipital bone becomes shorter (the slope becomes shorter), slanting horizontally, and the plane of the foramen magnum of the occipital bone is shifted upward, which is often combined with a flattened cranial base, making the posterior cranial fossa smaller;
2. Paracentral plunge
For occipital epiphyseal dysplasia, occipital condylar dysplasia, the slope is displaced backward into the posterior cranial fossa, and the slope is sunken so that the occipital scales are descended.
Symptoms
Congenital cranial base depression often appears gradually after middle age with symptoms of neurological dysfunction, often starting in adolescents, with age, degeneration of craniocervical joints and ligamentous laxity, it gradually develops into the craniocervical region of the joints of the unstable and cause the upper cervical medulla, the brainstem, the cerebellum, the posterior group of cranial nerves and vertebral basilar arteries insufficient blood supply of the symptoms, such as nystagmus, vertigo, ataxia, motor and sensory disorders of the limbs and trunk, and in severe cases, it can Respiratory depression and sleep apnea may occur.
Examination
1. X-ray film
Craniocervical lateral measurement of the dentate process of the cardinal vertebrae: 3 mm above the palato-occipital line (the line from the posterior border of the palate to the posterior superior border of the foramen magnum). The tip of the odontoid process in the craniocervical position is less than 10 mm from the diastasis line (the line joining the diastasis muscles on both sides).
2. CT and MRI (magnetic resonance imaging)
Combined subcerebellar tonsillar herniation malformation, midbrain aqueduct stenosis, and spinal cord cavernous disease may be detected.
Diagnosis
When the lateral atlantoaxial joints cannot be seen clearly on open radiographs, cranial base depression should be suspected. Chamberlain’s line measurement is the main imaging basis for the diagnosis of cranial base depression, which can be diagnosed when 1/3 of the dentate process exceeds this line (which is normally below this line) or exceeds this line by more than 3mm; the dentate process also exceeds Wackenheim’s slope line when the anterior and posterior diameters of the occipital foramen are less than 19mm (normally 35mm±4mm), or when the anterior and posterior diameter of the occipital foramen are less than 19mm (normally 35mm±4mm). When the anterior and posterior diameters of the foramen magnum are less than 19 mm (normal is 35 mm ± 4 mm), neurologic symptoms may occur.
Treatment
It is important to note that respiratory dysfunction may worsen or respiratory failure may occur during surgical treatment.
1. Cranial base depression is often combined with abnormalities of the dentate process, and the dentate process protrudes into the posterior cranial fossa, thus compressing the pontine medulla oblongata and cervical medulla, and most symptomatic patients can recover from cervical traction treatment with Halo ring.
2. Cranial base depression is often combined with Chiari malformation, such as the bone structure in the ventral compression of the medulla oblongata, in the posterior fossa of the cranium decompression should be performed before the ventral decompression, to relieve the compression of the ventral structure of the medulla oblongata; when there is a spinal cord cavity or effusion should be performed in the posterior fossa of the cranium decompression surgery, anterior decompression of the spinal cord after the cavity and effusion can be more often subsided, after the removal of the bone compression of the neurological symptoms can be improved.
3. Secondary depression of the skull base or skull base pressure trace is the result of bone softening, decompression should be combined with bone fusion or stent fixation and traction, the treatment of osteogenesis imperfecta, Paget’s disease, Haudju-Cheney syndrome is mainly the application of stent traction and fixation.